Oxidation protective coatings for vanadium alloys

ABSTRACT

A SLURRY FOR COATING VANADIUM BASE ALLOYS TO PROVIDE OXIDATION PROTECTION COMPRISING A METAL OF EITHER SILVER, TIN, OR COPPER, SILICON, AND A CARRIER SUCH AS NITROCELLULOSE LACQUER, TO WHICH ALSO MAY BE ADDED ALUMINUM AND COLUMBIUM.

United States Patent 3,575,735 OXIDATION PROTECTIVE COATINGS FORVANADIUM ALLOYS Dean B. Lawthers, Pittsburgh, Pa., assignor to theUnited States of America as represented by the Secretary of the Navy NoDrawing. Filed Oct. 9, 1968, Ser. No. 766,309 Int. Cl. C23c 9/00 US. Cl.1486 2 Claims ABSTRACT OF THE DISCLOSURE A slurry for coating vanadiumbase alloys to provide oxidation protection comprising a metal of eithersilver, tin, or copper, silicon, and a carrier such as nitrocelluloselacquer, to which also may be added aluminum and columbium.

BACKGROUND OF THE INVENTION The present invention relates to vanadiumalloys and more particularly to a slurry coating for vanadium basedalloys for providing resistance to oxidation.

Vanadium based alloys have a promising future, particularly in aerospaceapplications, as these alloys have low density, good ductility, andweldability. However, oxidation has been a serious problem as theformation of molten vanadium pentoxide upon exposure to air at therelatively low temperature of 1250 degrees F. results in catastrophicfailure. In order to overcome this weakness, various oxidationprotection systems have been studied for vanadium alloys. One promisingsystem involves a pack siliconizing treatment that leads to theformation of a surface compound with the base m tal, resembling theprocesses used to protect columbium and molybdenum.

SUMMARY OF THE INVENTION The present invention relates to a slurrycoating for vanadium based alloys and has several advantages. First,

DESCRIPTION OF THE PREFERRED EMBODIMENTS The surface of a specimen to begiven a slurry coating is first prepared by grit blasting, followed bypickling in a dilute acid bath. The specimen is then rinsed in acetoneand dipped in the slurry composition. The slurry composition might becomposed of silicon and a metal, such as silver, tin, and copper whichare suspended in a nitrocellulose lacquer. Commercially availablenitrocellulose lacquer can be used but it should be clear andunpigmented. Collodion is one type of commercial lacquer which can beused. The lacquer constitutes the liquid vehicle for the dispersal ofthe particulate silver, tin, or copper, and the silicon during coating.The lacquer must have an adequate green strength to hold the coating inplace after being air cured and during heat treating. The lacquerdecomposes during heating, leaving only a small amount of residue whichdoes not adversely affect the final coating.

A weight percentage of between 60 and percent silver to between 20 and40 percent silicon is considered to provide optimum processingconditions and this range tends to insure a sufiicient supply of eachmaterial throughout the coating to produce a uniform coating. In thisconnection, the silver is not a constituent of the final silicidecoating, but rather the vehicle in which the silicon may migrate to thesurface of the vanadium alloy to react with the vanadium to produce VSiThe silver is vaporized off during the heat treatment. Both tin andcopper have been shown to be effective substitutes for silver. The ratioof lacquer to powdered ingredients is not critical and will depend uponthe means employed in applying the coating to the specimen. For example,a ratio of about 5 parts powder to 1 part lacquer (by weight) might beemployed for a dip coating and a ratio of about 4 parts powder to 1 partlacquer might be employed for a spray coating.

The following examples exemplify the present invention:

EXAMPLE 1 A slurry was made with the following percentages ofingredients, by weight:

Silver 56.25 Silicon 18.75 Nitrocellulose lacquer 25.00

A test specimen of 60 percent vanadium, 39 percent columbium and 1percent titanium was grit blasted and then pickled in 25NHO 5HF-70H O,and then rinsed in acetone. The test specimen was then dipped in theslurry and allowed to air dry. The test specimen was then given adilfusion treatment in a stainless steel muffle in a tube type furnace.A vacuum was first pulled on the muffie and then an argon flow wasstarted. The diffusion treatment was conducted at a temperature of about1800 degrees F. for a period of four hours under a dynamic argonatmosphere. The resultant coating was between 0.001 and 0.002 inch inthickness with the desired silicide layer being about 0.0005 inch. In anoxidation test, the test specimen was heated to a temperature of 2200degrees F. and failure resulted in about 1 hour.

EXAMPLE 2 A slurry was made with the following percentages ofingredients, by weight:

Tin 56.25 Silicon 18.75 Nitrocellulose lacquer 25.00

A test specimen was cleaned, clipped, and diffused as in Example 1. Inan oxidation test, the test specimen was heated to a temperature of 2200degrees F. and failure resulted in about 1 hour.

EXAMPLE 3 A slurry was made with the following percentages ofingredients, by weight:

Copper 60 Silicon l5 Nitrocellulose lacquer 25 A test specimen wascleaned, dipped, and diifused as in Example 1. In an oxidation test, thetest specimen was heated to a temperature of 2200 degrees F. and failureresulted in about 1 hour.

3 EXAMPLE 4 A slurry was made with the following percentages ofingredients, by weight:

Silver 53.5

Silicon 17.9 Sodium fluoride 3.6 Nitrocellulose lacquer 25.0

EXAMPLE 5 A slurry was made with the following percentages ofingredients, by Weight:

Tin 53.5 Silicon 17.9 Sodium fluoride 3.6 Nitrocellulose lacquer 25.0

A test specimen was cleaned, dipped, and diffused as in Example 4, thatis, it was given a double-dipped treatment. In an oxidation test, thetest specimen was heated to a temperature of 2200 degrees F. and failureresulted in about hours.

EXAMPLE 6 A slurry was made with the following percentages ofingredients, by Weight:

Copper a 57.2 Silicon 14.2 Sodium fluoride 3.6 Nitrocellulose lacquer25.0

A test specimen was cleaned, dipped, and diffused as in Example 4. In anoxidation test, the test specimen was heated to a temperature of 2200degrees F. and failure resulted in about 10 hours.

EXAMPLE 7 A slurry was made with the following percentages ofingredients, by Weight:

Tin 48.75 Aluminum 18.75 Columbium 5.63 Silicon 1.87 Nitrocelluloselacquer 25.00

The test specimen was cleaned, double-dipped, and diffused as in Example4. In an oxidation test, the test specimen was heated to a temperatureof 2200 degrees F. and lasted longer than 10 hours before failure.

4 EXAMPLE 8 A slurry was made with the following percentages ofingredients, by weight: Tin 51.4 Aluminum 15.0 Silicon 8.6Nitrocellulose lacquer 25.0

The test specimen was cleaned, doubledipped in the slurry and diffusedas in Example 4. In an oxidation test, the test specimen was heated to atemperature of 2200 degrees F. and failure resulted in 30 hours.

In comparing the above-examples, the test specimens that were given adouble coating had a substantially increased oxidation life at atemperature of 2200 degrees F. For example, the specimens that weregiven a single dip coating failed in about one hour, while a secondcoating increased the oxidation life to 10 hours or longer.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. For example, satisfactorycoatings have been diffused at a temperature range between 1800 and 2000degrees F. for a period between 1 and 4 hours. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1. A high temperature oxidation resistance composition of matter forcoating vanadium based alloys consisting, by weight, of

between 48 and 68 percent of a metal selected from the group consistingof silver, copper, and tin, between 1.5 and 20 percent silicon,

between 15 and 25 percent aluminum, and

between 15 and 25 ercent nitrocellulose lacquer.

2. A method for applying an oxidation resistance coat ing to vanadiumbased alloys comprising:

coating 2. member formed of said vanadium base metal by dipping in aslurry consisting, by weight, of between 48 and 68 percent of a metalselected from the group consisting of silver, copper, and tin, between1.5 and 20 percent silicon, between 15 and 25 percent aluminum, andbetween 15 and 25 percent nitrocellulose lacquer, and

then diffusing said coating by heating said member in an inertatmosphere at a temperature between 1800 and 2000 degrees F. for aperiod of 1 to 4 hours.

References Cited UNITED STATES PATENTS 4/1959 Wainer 174 1/1965 Hubbard75134 ALFRED L. LEAVITT, Primary Examiner C. WESTON, Assistant Examiner

